The arm shaft of a sewing machine or sewing machining installation, e.g. an installation for the automatic high speed sewing of various patterns and materials, frequently must be driven at various speeds depending upon the requirements of the contour of the pattern to be stitched and it is not uncommon for a given stitching operation to require arm shaft speeds between a very slow minimum or even zero-speed to a maximum speed for straight seam simple stitch sewing.
The arm shaft of the sewing machine is customarily driven by an electric motor and the speed of the arm shaft can be controlled by electrically operated clutch and brake means, controlling the torque transfer to the arm shaft from the motor or the rotation of the arm shaft.
It has been proposed heretofore to couple the shaft or the motor or some other element rotating synchronously with the arm shaft with a tachometer or other means for providing an analog signal or even a means for providing a digital signal representing the speed of the arm shaft, to provide a controller with an input representing a set point value of the speed of the arm shaft, and to digitally compare the set point and actual speed values to produce an error signal which is applied to the brake clutch combination to modify the arm shaft speed in accordance with the set point value. The set point value generator was generally a knee- or pedal-operated element.
Such means is provided, for example, in the system described in German patent document--open application DE-OS No. 24 15 359 which provides for the stopping of the arm shaft of a sewing machine in a predetermined angular position. In this case actual value measurement of the arm shaft speed is effected in an analog manner with a tachometer or tachogenerator connected fixedly with the arm shaft. The analog set point value is generated by short-circuitable resistance elements and after the set point-actual value comparison, the control magnitude is applied as a voltage with variable keying ratio to the brake/clutch comparison. Obviously, this system does not provide full digitalized speed control and time consuming setting and initializing of the potentiometer in the control system cannot be avoided.
A positioning drive with at least one 8-bit microprocessor is also known in the literature, e.g. from German patent document DE-OS No. 29 38 040. Here the actual value of the arm shaft speed is generated by an optoelectronic increment generator fixed to the arm shaft to provide a digital output while the set point value in analog form is obtained with a resistor/condenser circuit operating a voltage-controlled oscillator and after set point-actual value comparison the control magnitude in digital form is applied with variable keying to the brake/clutch combination. Here again potentiometer initialization or zeroing is required.